This invention relates generally to a reinforcing support structure for aircraft structural panels of the skin-stiffener type, and more particularly to stiffeners mounted on aircraft panels.
Over the past several years aerospace design engineers have been challenged to address ever higher payload requirements. This has occurred in parallel with weight reduction and other structural efficiency goals that have driven the industry. Ever present safety and structural integrity demands have led the industry to look for more efficient and effective structural members and configurations that are more robust while addressing weight concerns.
Generally speaking, a stiffened aircraft panel""s load-carrying capability is directly related to the type and strength of the stiffener typically installed on one side of the panel. The current approach within the industry to meet higher competitive demands has ranged from resorting to sophisticated alloys including extrudable aluminum and other alloys. It has also included such approaches as using specialty fiber reinforced composite structures, and increasing the depth of conventional stiffener designs as well as the yield strength of the material used in making the stiffeners. In order to appreciate the uniqueness and novelty of the current invention, a better understanding of the current state of the art in addressing the above airframe requirements follows. The first, and most common, approach taken by the industry in addressing the higher requirements has been to make conventional stiffeners out of heavier or higher strength materials. These traditional stiffener designs include the C-channel stiffener and Z-shaped stiffener, as well as the hat-shaped stiffener. Heavier gauges such as 0.055 inch min. (17 gauge) to 0.070 inch min. (15 gauge) material are now common. The use of thicker material has not only lead to greater tooling and handling costs, but also as will be shown, has had the effect of creating other major problems simultaneously.
The aircraft structural panel including any attached stiffeners is a system of parts interacting with each other as they are acted upon by combinations of pressures and in-plane as well as bending loads. Currently, stiffened-panel aircraft structures found in the wings, empennage, and fuselage of aircraft are typically constructed using aluminum or fiber reinforced composite skins with aluminum stiffeners and titanium members, or stiffeners using fiber reinforced composites.
These panels may also be supported by metal stiffeners to provide greater support as the stiffened panel system sustains bending moments as well as in-plane and pressure forces during service. However, an incompatibility occurs when relatively thick, stiffer sections, i.e., stiffeners made of 0.55 inch min. to 0.070 inch min. material are joined or fastened to thinner, less stiff sections, i.e., stringers made of 0.023 inch min. to 0.038 inch min. material. The area where these two sections are joined is an area of load transfer and thus of relatively higher stress. The reason for this is that the stiffer section resists conforming to the deformation of the less stiff stringer sections as loads are increased. The result is that one part of the system, the stringers, try to slide relative to another part of the system, the stiffener. This may result in early failure of the system, such as by buckling of the stiffener or of the stringer. This is due to in-plane compressive loads that result from the constraint that the stiffener imposes on the adjacent members as the loads are increased. Because of the increased stress at the joining area, manufacturers have been forced to modify parts of the stiffened panel to offset this effect. For example, because the use of heavier stiffeners increases the shear load through the joining area or fasteners, especially on the outer extremes of the panel width (near the panel edges), heavier panels and mounting members have had to be introduced. Still another approach to alleviate the problem has been to use additional shear ties or fasteners. This has been implemented in an attempt to reduce the high local in-plane compressive stresses that the heavy stiffeners may impose on the panel skin. However, this approach is undesirable because by increasing the number of parts, it increases the complexity and cost of the system.
This approach requires still heavier stiffeners, since the stiffener failure risk is somewhat reduced when it acts as an independent component rather than as part of a fully integrated system. Another drawback to additional shear ties is that it requires substantially more parts and installation time.
The second approach generally taken by the industry is to make the current hat-shaped and C-channel stiffeners deeper and out of thinner, yet higher yield strength material. This offers the advantage of reducing in-plane stress as noted above while at the same time increasing bending stiffness due to the deeper configuration. However, this approach has major disadvantages.
First, the thinner materials used in these traditional stiffener configurations make these stiffener sections more susceptible to edge stress concentrations. The conventional C-channel, Z-shaped, and hat-shaped stiffeners have a xe2x80x9cblade edge.xe2x80x9d This edge is very susceptible to imperfections in the sheet material along this edge as well as to damage during manufacture, shipping/handling and installation. These imperfections along the blade edge become stress concentration points or focal points at which failure of the stiffener can initiate. A more detailed description of this failure initiation follows.
Even the most perfect, smooth edge of the conventional stiffener will experience a very localized point of high stress gradient due to the characteristic edge stress concentration associated with open sections under bending loads.
Thus, initiation of an edge xe2x80x9cbulgexe2x80x9d or xe2x80x9ccrimpxe2x80x9d on a perfect smooth edge is nothing more than the creation of an edge imperfection that is large enough to grow or xe2x80x9cpropagatexe2x80x9d easily. It is significant that this stress concentration may be made worse by the presence of any relatively small local edge imperfections, even those on the order of size of the thickness of the stiffener material itself.
These imperfections near the edge can be in the form of edge notches, waviness (in-plane or out-of-plane), local thickness variations, local residual stress variations, or variations in material yield strength. Where multiple imperfections occur together, they may all compound together to further increase the stress concentration effect, and thus lower the load level at which failure is initiated. Thus, the existence of any edge imperfections in a conventional stiffener has the effect of enhancing an already established process of failure initiation.
Second, all the above conventional stiffeners, when manufactured out of relatively thin sheet materials are more susceptible to buckling due to the reduced thickness. Buckling is an instability in a part of the stiffener associated with local compressive or shear stresses. Buckling can precipitate section failure of the stiffener. This in turn causes a stress concentration in the adjacent panel skin near the buckled stiffener section, which may cause the stiffened panel to fail.
Finally, some thinner conventional stiffeners can experience xe2x80x9crollingxe2x80x9d when placed under load. Rolling may be caused when the shear stresses within the stiffener result in a net torque about the centroid of the thin walled cross-section thus causing the cross-section to twist, possibly making the stiffener unstable. Another cause of rolling is the curvature of the panel itself that is induced by in-plane or pressure loads that are imposed upon the stiffened panel. Some airframers have increased the cross-sectional length of the flange furthest from the panel skin of the conventional C-channel stiffener in their attempts to solve the rolling problem, but have been met with only marginal improvement. This is because the increased flange length has had the simultaneous effect of increasing the distance from the centroid to the shear center of the channel. Additionally, increasing the cross-sectional flange length caused difficulty in accessing the fastener areas used in mounting the C-channel to the stiffened panel.
The use of solid bulb flange-edges for extruded stiffeners has surfaced as another attempt to address aircraft stiffener needs. However, due to the relative material inefficiency of providing such a solid, mass-intensive edge section, as well as the limitation to specific extrudable aluminum alloys, this solution has not gained extremely wide usage. Because of the higher load requirements of current aircraft, and problems such as the fastening of relatively thick sections to sections relatively less thick, there is a need within the industry today for a new stiffener configuration that can address all of the above mentioned drawbacks and shortcomings of the present state of the art, is suitable for use with substantially all standardized stiffened panel structures, and can be made on a cost-effective basis.
The present invention alleviates and overcomes the above-mentioned problems and shortcomings of the present state of the art through a novel aircraft structural panel stiffener. The novelty and uniqueness of this invention are that it: 1) may be made of thinner material to reduce the in-plane stresses found in the joining area, 2) resists loads adequately to meet new requirements, 3) is resistant to buckling and rolling, 4) effectively addresses edge stress concentrations by modifying the blade edge to an area of relatively low stress, and 5) can be manufactured cost effectively by using conventional manufacturing methods.
This novel invention may be described as a substantially reconfigured or stabilized J-stiffener having a mounting or stabilizing flange. It should be noted here that due to their extreme susceptibility to rolling, conventional J-stiffeners are seldom used in aircraft structural panels. The unexpectedly strong synergisms of the unique characteristics found in the stabilized J-stiffener not only address the above problems, but simultaneously obtain significant material savings. More particularly the synergisms may be described as follows.
The instant invention has substantially redistributed material at critical locations as compared with conventional stiffener configurations. This material redistribution has the effect of altering considerably the behavior of the stiffener as compared with conventional J-stiffeners and other stiffener configurations.
The material redistribution required to accomplish these collaborative effects is accomplished by having specifically placed free edge portions, which are turned inwardly to define tubular beads or curls along the free edges. Moreover it is not just the presence of the tubular bead or curl that enables the substantial level of synergism, but the discovery of specific ratios of curl diameter to other stiffener dimensions that maximize these synergisms even to the extent of obtaining significant weight savings.
Two sets of synergisms combine to make the present invention successful. The first set of synergisms is directly related to the ratio of the diameter of the curl to the stiffener section flange length and web length. Each tubular bead has a cross-sectional dimension which when combined in specific ratios with other stiffener dimensions substantially maximizes the moment of inertia of the overall section about the horizontal and vertical axes with a minimal use of material. Moreover, the tubular bead size specified by these same ratios has the effect of altering the characteristic failure mode normally associated with the free edge stress concentration for conventional stiffeners as described above. Finally, the cross-sectional dimension of the tubular beads of the stabilized J-stiffener make the novel stiffener less sensitive to edge imperfections and damage because the blade edge has now been placed in a position of relatively benign stress levels so that imperfections or damage to the tube or edge region have to be on the order of size of the diameter of the curl in order to have significant detrimental effect to the stiffener section.
Having established the above ratios, a second set of synergisms was discovered by directly combining the above with specific ratios of the stiffener""s cross-sectional web dimension to cross-sectional flange dimension. The compounding effect of the first set of synergisms with this additional set of ratios makes the stabilized J-stiffener more resistant to rolling and buckling and thus avoids the problems that plague deeper conventional structural panel stiffeners using thinner gauge material. Additionally these compounding synergisms make this stiffener unique in that stresses are now more evenly distributed in the flanges thus making the stiffener more stable and less sensitive to dimensional imperfections. Because of these cooperative effects, the stabilized J-stiffener demonstrates its uniqueness and efficiency in using thinner gauge material to reduce in-plane stresses found in the joining area, thus allowing the panel and stiffener to work together as a more cohesive system instead of as individual components.
Because the stabilized J-stiffener effectively addresses the problem of in-plane stresses in the area nearest the panel skin, the use of fewer fasteners may be considered. Thus, the airframer may now uniquely rely upon a single stiffener design to address the stiffening of a wide variety of structural panel constructions.
When compared to conventional stiffeners on the market today, the stabilized J-stiffener uses substantially thinner material while obtaining better resistance to service loads. Thus, even though additional slit width (width of the sheet of material from which the stiffener is made) is required to reposition needed material, the use of thinner gauge material more than offsets the additional slit width, bringing overall material savings as high as 15% in some instances. This innovation in system configuration represents an additional cost savings for the airframer, since material cost is a significant portion of total manufacturing cost of stiffened panel structures. Thus, this unique and novel stiffener is cost effective.
For manufacturing process cost efficiency, the tubular bead is preferably an open-section bead, meaning for example that the sheet material may be formed in an almost complete bend or curl, but the curl need not be closed near its outer edge, such as by welding, bonding, or joining. A closed section tubular bead would work equally well, at a slightly higher fabrication cost.
This edge feature is discussed in more detail as follows. The mounting flange curl and the trough curl are tubular features, preferably open-sections, that are made by shaping the free edges or edge marginal portions of the stiffener cross-sections into an elliptical, preferably circular, cross-sectional shape. As used herein, a circular cross-section is considered to be a special case of an elliptical cross-section. The term xe2x80x9ccharacteristic diameterxe2x80x9d refers to a constant diameter in the case of a circle, while other elliptical shapes will have major and minor axes or diameters, with the minor axis or diameter being the xe2x80x9ccharacteristic diameter.xe2x80x9d
Even though some configurations of a slightly non-circular elliptical shape may be more desirable in some applications, the circular cross-section is generally preferable, because it is simpler to manufacture, while still achieving the desired benefits to a significant degree.
It is important to contrast the edge curl approach against other possible edge treatment approaches by noting that the dimensional order of size effect related to imperfections or damages described above for the curl can not be achieved by simply folding the edge over, either once or multiple times, because in this case the characteristic dimension with respect to the local stresses, will be defined by the fold edge diameter and not by the length of overlap of the fold. This is because the overlap direction is transverse to the edge and quickly moves out of the peak stress region, and because the edge fold diameter defines the maximum distance over which the edge stresses may be effectively spread.
The elliptical or circular open-section tubular shape or xe2x80x9cedge curlxe2x80x9d is contrasted to tubular sections of rectangular cross-sectional shapes, including folded edges, and to open-section tubular shapes of softened corner polygon cross-sectional shapes in that the characteristic diameter with respect to the local stresses, will be defined in each of these other cases by the fold diameter or by the softened corner diameter nearest to the stiffener edge, as opposed to the overall diameter of the edge curl section.
It may be noted that in this context a rectangular or polygonal cross-section with very softened corners or curved sides is in effect an imperfect ellipse or circle.
In some instances, quasi-elliptical or quasi-circular cross-sections, imperfect ellipses, and imperfect circles, in the form of polygon cross-sections with very softened corners may function adequately, but may also be more difficult to fabricate and will be less effective than a generally circular curl. In other cases, the tubular edge may be formed by lapping the edge over itself one or many times in order to achieve specific design objectives within the teachings of the present invention.
Other variations are obtained by including local offsets or adding material locally such as by bonding or welding strips of material or high strength fibers or wires. Still other variations include local modifications to the material such as by heat, electromagnetic, chemical, or deformation treatment of the tubular bead cross-section or of adjacent regions. In spite of the potential for additional fabrication costs, some of the above variations may at times be desirable, for example in local regions where the designer desires local regions of modified cross-sectional shape for space claim, interfacing, or joining reasons. In some applications the curl may be formed by turning the edges through an arc of up to 360 degrees, 720 degrees, or even more, so that the edge loops over one or more times on itself, in order to concentrate mass locally or to address other design objectives. In these cases manufacturing economy and complexity are also considerations.
The resulting synergistic effect of the stabilized J-stiffener""s material efficiency in obtaining the desired bending rigidity or moment of inertia, the alteration of the characteristic failure mode, the reduction in sensitivity to edge imperfections and damage, resistance to buckling and rolling as well as the ability to spread stresses more uniformly has the same degree of compounding advantage as the conventional stiffener""s compounding disadvantage of low resistance to buckling and rolling combined with sensitivity to relatively small edge or dimensional imperfections.
Accordingly, it can now be appreciated by those versed in this art, that the novel stabilized J-stiffener of the instant invention provides a solution to the problems that the airframe stiffened panel art has sought to overcome. Areas of viable application include stiffened panel applications in wing, empennage, and fuselage areas, where they may be used as frame structural members. In these applications the stiffeners are usually installed such that they project in a direction substantially perpendicular to the surface of the panel skin with which they are associated.
In summary, the stabilized J-stiffeners of the present invention having inner stabilizing flanges are uniquely designed to be compatible with substantially all standard stiffened panel configurations, thereby significantly reducing the number of stiffeners and panel frame members that airframers must carry in their inventories and employ, to permit more stringent design criteria to be met, and to permit this to be done without major modification of other associated hardware.
The following description of the present invention may incorporate dimensions that are representative of the dimensions that will be appropriate for most commonly found stiffened panels. Recitation of these dimensions is not intended to be limiting, except to the extent that the dimensions reflect relative ratios between the sizes of various elements of the invention, as will be explained where appropriate.
It is an object of the invention to provide stiffeners for aircraft panels of a minimum weight while maintaining strength requirements.